1. Field of the Invention
This invention relates to a lapping abrasive for polishing or lapping various products made from metal materials, glasses, synthetic resins and so forth, and more particularly to a lapping abrasive suitable for lapping memory hard discs, magnetic heads, photomagnetic discs made by these materials or a magnetic thin-film coated on the photomagnetic discs, and to a method for producing said lapping abrasives.
2. Prior Arts
Conventionally known as lapping abrasives for general metal materials, glasses and synthetic resins are aluminum oxides(alundum and white alundum), silicon carbides(carborundum and green carborundum), boron carbides, cerium oxides, chromium oxides, tin oxides, iron oxides, titanium oxides and diamonds.
These lapping abrasives are finely crushed and classified into a given grain size and an abrasive composition in a slurry form is prepared for a lapping purpose by adding water thereto.
Recently, lapping abrasives have been used for various purposes so that superior processing characteristics have been required. The above mentioned general abrasive compositions, however, have not attained satisfactory results.
Slurries of conventional lapping abrasives prepared by adding water to the above abrasive materials are likely to cause micro-scratches, pin holes(pits) or orange peels on a lapped surface. Orange peels are caused by a defect of the crystal surface induced by various factors which have not been fully clarified. In order to prevent the occurrence of micro-scratches or orange peels, both a high level of skill and considerable time are required to control grinding factors such as the concentration of abrasive grains in an abrasive composition, grinding time or grinding load. Accordingly, it has been difficult to prevent the occurrence of the above deficiencies for both technical and economical reasons.
Lapping abrasives are generally evaluated by the grinding rate, that is, the rate of stock removal in a given time and the surface roughness of the finished work. In case the grinding rate is higher and the surface roughness is closer to the desired roughness, lapping abrasives are valued to be superior.
The grinding rate is improved when abrasive grains of a particle size are used, but the surface roughness becomes more coarse. On the other hand, fine abrasive grains can make the surface roughness fine and easily obtain the desired roughness, but the grinding rate becomes low. Therefore works were conventionally lapped to the desired roughness by first using coarse abrasive grains and then using fine grains with the particle size decreasing gradually. In this case, however, works must be rinsed every time abrasive grains are changed. As a result, the loss of grain is great and the grinding process becomes complicated and takes much time. This is undesirable from the industrial point of view.
In order to solve the above problems, there have been provided some abrasive compositions.
Japanese patent publication No. 53-3518 provides an abrasive composition comprising water, abrasive grains such as aluminum oxide, cerium oxide etc. and 0.1-15 wt% of acidic compounds such as polyaluminum chloride, aluminum nitrate, aluminum bromide, etc. as grinding accelerator. When said abrasive composition is used for lapping a synthetic resin, the grinding rate is improved and coarse grinding grains are not required. Accordingly a high quality of the ground surface is obtained which is free from grinding defects such as scratches, orange peels etc. There are, however, such defects that pollution problems are caused by waste water which includes strong acids such as nitric acids etc., and, in addition besides such strong acids rust the grinding machine and metallic jigs and further chap the hands of the operator.
Japanese patent publication No. 53-44273 provides an abrasive composition for general metal materials comprising silica sol abrasives and an acidic compound such as hydrogen peroxide, sodium peroxide, sodium perborate etc. said abrasive composition, however, is strongly acidic and Thus the same problems occur as in the case of the above abrasive composition. Besides, silica sol abrasives are lower in hardness than aluminum oxide abrasives, so that the grinding rate is low.
Japanese patent application laid open under No. 53-41886 proposes to use abrasive grains composed of secondary particles of 1-200 .mu.m which are prepared by bonding primary particles of 0.1-0.5 .mu.m comprising diamond, cubic boron nitride, aluminum oxide and silicon carbide using a binder, whereby a superior grinding rate and a fine ground surface can be obtained.
But, a complicated process is required to granulate and bond the primary particles to the secondary particles and besides it takes much time to regulate the particle size uniformly.
Japanese patent application laid open under No. 60-108489 teaches a lapping method suitable for grinding the surface of a magnetic medium layer of a memory hard disc comprising first supplying an aqueous slurry of aluminum oxide powders containing such oxides as sodium hypochlorite between the surface of a magnetic medium layer and a grinding pad which are placed in sliding engagement in a relative manner and second supplying a colloidal slurry of aluminum oxide as well as the above aqueous slurry.
Said two-step method, however, takes much time and nevertheless the stock removal rate is not high and the grinding rate is low.
Japanese patent publication No. 64-436 discloses an abrasive composition wherein the above defects are almost solved. Said composition comprises water, aluminum oxide abrasives, i.e. .alpha.-aluminum oxides obtained by calcining granular boehmite at 1150.degree. C. for 3 hours, and 1-20 wt % of nickel sulfate as grinding accelerator. Said composition is neutral or weakly acidic. A plastic surface, electroless nickel plating surface of a memory hard disc, alumite or aluminum surface can be ground mechanochemically by both a mechanical processing with abrasive grains and chemical etching with a processing fluid containing a grinding accelerator. As a result, a superior ground surface and grinding rate can be obtained. Besides said composition is the neutral or weakly acidic which is poor in oxidative effect, so that the grinding machine and metal jigs are hardly rusted and the operator's hands are hardly chapped. Said composition, however, contains .alpha.-aluminum oxides alone as abrasive grains, so that fine primary grains must be used in order to obtain a given surface roughness. Thus it is not satisfactory in the grinding efficiency.